The present invention relates to a process for the production of austenitic stainless steel strips having, as cast, a good weldability, through the solidification thereof in a mould with counterrotating rolls of a continuous casting apparatus. Further, the present invention relates to an austenitic stainless steel strip so obtainable through said process and suitable for the production of welded tubes.
Austenitic stainless steels are known to provide an excellent corrosion and oxidation strength, together with good mechanical properties. In fact, these kinds of steel are often employed in the production of tubes starting from flat products derived from hot-rolling followed possibly by cold-rolling processes.
Generally, thin stainless steel strips are obtained by a conventional process comprising the continuous casting of slabs, followed possibly by a grinding operation, slabs heating to 1000-1200xc2x0 C., hot-rolling, annealing, possibly followed by cold-rolling, final annealing and pickling.
This process requires a large energy consumption both for the slabs heating and for the material processing.
On the other side, the continuous strip casting process is a recent, still developing technique, shown, for instance, in xe2x80x9cRecent developments of Twin-Roll Strip Casting process at AST Terni Steelworksxe2x80x9d of the authors R.Tonelli, L.Sartini, R.Capotosti, A.Contaretti; Pro. Of METEC Congress 94 Dusseldorf, Jun. 20-22 1994, by which it allows thin strips to be produced directly as the cast product and thus avoiding the hot-rolling operation.
In order to obtain austenitic stainless steel strips suitable to be used as cast, it is necessary to operate on the primary solidification procedure. In fact, the primary solidification structure is subject to changes from austenite to ferrite (xcex4-ferrite) depending on the steel chemical composition and on the cooling rate during solidification.
The formation of a suitable quantity of xcex4-ferrite during the solidification process is crucial to avoid cracks to be formed in the cast strips. The presence of xcex4-ferrite is also advantageous for the successive weldability of the strips to avoid cracks due to the heating. On the other hand, an excess of xcex4-ferrite at the welded joints, can involve risks concerning corrosion strength and ductility.
Various control procedures for continuous casting of austenitic stainless steel strips are known in the art. For instance, EP 0378705 B1 discloses a process for the production of stainless steel thin strips aimed at obtaining a good surface quality by controlling the differential cooling rate at a high and low temperature and by controlling the xcex4-ferrite volume percentage in the resulting cast product.
On the other hand, EP 043182 B1 discloses a process for the production of stainless steel strips having excellent surface qualities based on the main choice of holding the obtained strip at specific temperatures for fixed periods of time.
However, the above processes aim at improving the final product surface quality, and do not teach a method for obtaining a product having excellent weldability.
Therefore, the present invention provides a process for the production of austenitic stainless steel strips, by means of the continuous casting technique in a mould with counterrotating rolls, that it aims at obtaining excellent weldability properties on the strips as cast.
Another object of the present invention is to provide austenitic stainless steel strips, obtained with the above process, and having excellent weldability properties as cast and being suitable to be used in the production of welded tubes.
Thus, subject of the present invention is a process for the production of austenitic stainless steel strips having, as cast, good weldability, comprising the casting operation in a mould with twin counterrotating rolls of a continuous casting apparatus, of a strip having thickness comprised between 1 to 5 mm, and having the following composition in percent by weight:
Cr 17-20; Ni 6-11; C less than 0.04; N less than 0.04; S less than 0.01; Mn less than 1.5; Si less than 1.0; Mo 0-3; Al less than 0.03; and wherein Ti, Nb, Ta are provided in the strip so that:
Ti+0.5(Nb+Ta) greater than 6C-3S with the proviso that Ti greater than 6S; or
Nb+Ta greater than 12C with the proviso that Ti less than 6S;
being, in every case, Nb+Ti+Ta less than 1.0%; the remaining part being Fe and impurities, and having a dendritic solidification microstructure with an average grain size, measured on a cross-section parallel to the strip surface, comprised between 30 and 80 xcexcm, and having a xcex4-ferrite volume percentage comprised between 4 and 10%, calculated by the formula:
xcex4-ferrite=(Creq/Nieqxe2x88x920.728)xc3x97500/3
wherein:
Creq/Nieq=[Cr+Mo+1.5Si+0.5Nb+0.25Ta+2.5(Al+Ti)+18]/[Ni+30(C+N)+0.5Mn+36];
wherein the element symbols represent their weight percentage in the whole composition.
Further, according to the present invention, the process provides possibly the heating of the strip to a temperature comprised in the range from 900 and 1200xc2x0 C. for a period of time less than 5 minutes.
Furthermore, subject of the present invention is an austenitic stainless steel strip obtainable with the abovementioned process and suitable to be used in the production of welded tubes.
AccordIng to the invention, the austenitic stainless average grain size in the range from 30 to 80 xcexcm.
Further, the absence of central segregation of elements such as C, Cr, Ni, confers to the material homogeneity of properties together with the moderate grain size, being very important for both molding and welding operations.
The strip as cast shows a much lower residual strain-hardening ratio compared to that of a strip hot-rolled by a common work cycle and therefore does not require any stress relieving heat treatments before being used in molding operations.
The present invention has the further advantage that the resulting strips provide a suitable material to be welded for the manufacture of welded tubes not requiring final thermal treatments.
Another advantage of the present invention lies in that the resulting austenitic stainless steel strip, possibly when containing elements such as Ta, Ti, Nb, shows no grain edge dechromizing effect due to chromium carbide precipitation, therefore providing an improvement in corrosion strength and ductility of the welded portion.